Robotic systems, as understood in the industrial sense, generally consist of a robotic arm or manipulator capable of performing gross movements and a device known as an end effector which may either resemble a conventional machine tool or may be a separately controllable device capable of precise manipulation and/or grasping. Such robotic systems also generally include a computing system to control the articulation of the arm and end effector. A major class of end effectors generally capable of precise manipulation and/or grasping can be analogized to the human hand and, as such, usually contain controllable fingers capable of being manipulated to perform desired tasks. Preferably, these fingers should also be capable of sensing the force with which they are contacting an object and provide data for feedback control of the manipulation. This invention is directed towards an improved class of controllable robotic fingers along with optional but preferable force sensors, and the actuation, transmission and control systems for said fingers.
Although a number of finger designs exist, the class of fingers to which the present invention is directed are those designs allowing both dexterous manipulation and enclosure grasps of objects intended to be held securely by a robotic finger or in a robotic hand. Different types of robotic fingers may be distinguished by at least four features. These are: (1) the number of degrees of freedom of the finger; (2) the number of controlled actuators; (3) the type of transmission connecting the finger joints to the actuators; and (4) the method of determining the force applied by the finger.
It has now been shown that in order for a robotic finger to possess effective manipulating and grasping capabilities, it is required that the finger consist of at least three mechanical joints or links. See, The Design of a Lightweight Self-Contained Mechanical Finger, S. Leaver, 1986 p. 2, incorporated herein by reference. In prior configurations, one actuator and sensor were used to drive and control each joint of a robotic finger. See, Leaver, supra. pp. 2-7. In such configurations, the finger may be described as having three degrees of freedom and three controlled actuators. There are severe disadvantages to robotic fingers of this configuration. First, the weight of three actuators reduces the effective payload capacity of the robotic arm. Further, the control of three actuators adds to the size and complexity of the control system when the coordinated motion of one or more fingers which comprise a robotic hand is undertaken.
Previous finger designs have either sacrificed weight and control system computing resources for the sake of dexterity by increasing the number of actuators and sensors, or have achieved a lightweight finger with a simplified control algorithm which suffered from the lack of dexterity required to perform grasping motions. An example of the former design is the Utah/MIT hand while the latter type of design is embodied in the Pennsylvania Articulated Mechanical Hand (PAMH). See, S. Leaver, 1986 supra. at p. 2. Due to these shortcomings, it is apparent that the previous designs have been in need of substantial improvement.
Previous finger designs have also proved to be inadequate in terms of the type of transmission system which connects the finger joints to the actuators. Typically, finger joints have been driven by arrangements of pulleys and cables analogous to the tendons of a human finger. This type of system, as applied in previous robotic fingers, presents major disadvantages. First, the arrangements of the pulleys and tendons, as well as the exclusive reliance upon these mechanisms in the transmission system, has resulted in finger designs with an unnecessarily high degree of complexity and/or excessive weight, since individual tendons are generally attached to separate actuators. Although it is possible to reduce the weight in the vicinity of the hand by mounting the actuators at some remote location, this solution increases the mechanical complexity of the transmission system and requires special adaptations and modifications to be made to the robotic arm in order to accommodate the remote actuators.
Finger designs which utilize a screw/cam combination to manipulate one of the finger joints have also been disclosed. See, S. Leaver, supra. at p. 2. This type of finger articulation has been limited to non-grasping, two joint/two degree of freedom fingers. His design also lacks sensor means to determine the force imparted by the finger upon the object being manipulated.
If the finger joint torque can be determined by means of sensors, then finger force can possibly be controlled. The sensors can provide feedback to a tactile control system, increasing the usefulness of the finger as a manipulating and grasping device. Designs based on tendon and pulley transmission systems generally have computed joint torque by measuring tendon tension. The addition of an accurate torque measuring device can add to the transmission system compliance, or the measure of deflection the system will undergo when a static load is applied to the finger. Any successful embodiment of a tactile feedback sensing system must measure tendon tension accurately and effectively in an industrial environment, without adding a component of compliance to the transmission system which is above the desirable limits for such a system.